An essential operation in communicating by satellite is the acquisition (locating) of the satellite by the earth terminal antenna and the subsequent tracking of the satellite. Initial acquisition depends upon an exact knowledge of the position of the satellite. In combination with the geographic location of the earth terminal, knowing the position of the satellite enables you to compute accurate antenna pointing
information. The degree of difficulty in locating and tracking a satellite is determined largely by what type orbit the satellite is in.
The locating and tracking of a synchronous satellite is relatively simple. This is because the satellite appears to be stationary. Locating a near-synchronous satellite is also relatively simple because of the slow relative motion of the satellite However, the movement of a near-synchronous satellite is enough that accurate tracking is required to keep the narrow beam antenna pointed toward the satellite. Satellites in medium altitude circular orbits or in elliptical orbits are more difficult to acquire and to track because
of the rapid changes in position.
To acquire and track a satellite in space, the earth terminal antennas must be provided with very accurate pointing information. Antenna pointing information is based upon the orbital prediction of the satellite. This information is derived from an EPHEMERIS table. This table provides the coordinates of a satellite or a celestial body at specific times during a given period. After you know the ephemeris data of a satellite, you can predict for any given location the apparent track of the satellite as viewed from that
location.The constants defining an orbit are initially obtained by the process of tracking. At the time of launch, the rocket is tracked by radar from lift-off to orbit and then until it passes out of sight. Tracking data obtained in this way is sufficient for making rough predictions of the orbit. These predictions are made rapidly with a computer and sent to tracking stations all over the world. These other tracking stations watch for the satellite during its first trip and record additional data. During the first week of orbiting, tracking stations all around the world are obtaining progressively more accurate data concerningthe Satellite. This data is put into a computer where corrections of earlier estimates of the orbit are made. Once the initial predictions are complete and the satellite link becomes operational, very little change in these calculations is made. The orbits of a satellite will change slightly over a period of time; however, these changes are so gradual that predictions will be accurate enough to be used for weeks or even months without further corrections. When the orbits are known precisely, an ephemeris can be calculated for each satellite of the system.
Antenna Pointing
Antenna pointing instructions for each satellite must be computed separately for each ground station location. A satellite that bears due south of station A at an elevation of 25 degrees may simultaneously bear due southeast of station B at an elevation of 30 degrees. Antenna pointing instructions are determined by taking into consideration the orbital prediction and the latitude and longitude of eachground station. To establish radio contact with a satellite, the ground station needs to know the bearing and elevation of a satellite. This allows the antenna to be properly pointed.
Acquisition
The acquisition of satellite signals by a ground station equipped with large antennas and operated at microwave frequencies places severe requirements on the system. Several factors must be considered.
These factors are discussed below:
FREQUENCY CONTROL: The frequency of a radio signal received from a satellite is not generally the exact assigned down-link frequency. This variation depends upon the type of orbit of the satellite. The greatest frequency variations in signals from satellites occur in medium altitude circular or elliptical orbits. The smallest frequency variations occur in signals from satellites in near-synchronous or synchronous orbits.
Tracking
When a particular satellite has been acquired, the earth terminal antenna will track that satellite for as long as it is used as a communications relay. Several methods of tracking are in actual use.PROGRAMMED TRACKING: In programmed tracking the known orbital parameters of the satellite are fed into computation equipment to generate antenna pointing angles. The antenna pointing angles are fed as commands to the antenna positioning servomechanisms. These point the antenna in the required direction. The amount of data and computations involved in using programmed tracking is extensive. These are a result of the antenna mount flexing and atmospheric and ionospheric bending of
radio waves. Because of these uncertainties, programmed tracking is not used extensively.
AUTOMATIC TRACKING: In automatic tracking, the equipment generates antenna pointing information by comparing the direction of the antenna axis with the direction from which an actual satellite signal is received. Automatic tracking systems track the apparent position of a satellite. The direction of arrival of the radio signal and the real position of the satellite is not required. The automatic tracking system uses a servomechanism to move the antenna. Once the satellite has been located, the
servomechanism generates its own pointing data. This eliminates the requirement for continuous datainput and computation.
SATELLITE OUTAGE TIME: The satellite outage time specifications allow for stewing
(moving) the earth terminal antennas, acquiring the satellite signal, and checking for circuit continuity at HAND OVER. (Hand over is the period of time for one earth terminal to yield control to another as a satellite moves out of its area of coverage.) This hand over period represents an outage time. If the control terminal is unable to hand over to another terminal within a specified time, other arrangements are made.
For example, control may be retained or transferred to another terminal within the coverage area. There are several reasons why a terminal may be unable to assume control on time; these reasons may combine to increase the outage time. The difference of drift velocities of the satellites leads to bunching within a coverage area. This causes gaps in coverage and increases outage times. When two or more satellites simultaneously occupy the same space of the terminal antennas, they will interfere with each other. This prevents reliable communications. Other factors leading to increased outage times are SATELLITE-SUN
CONJUNCTION (increased noise while the satellite passes near the sun), SATELLITE ECLIPSE (absence of power from solar cells), and satellite failures. The distribution of outage times is a complicated function of time and earth-station locations. With careful coverage coordination, maximum communicationseffectiveness is obtained.
There are different sites providing Real Time satellite tracking for all those who are interested in it. They are provided below:
1) http://science.nasa.gov/iSat/
2) http://www.n2yo.com/
3) http://www.satview.org/
There are also applications for tracking satellites:
1) http://science.nasa.gov/realtime/jtrack/3d/JTrack3D.html/
2) http://www.stoff.pl/
3) http://www.nlsa.com/
